In certain water treatment applications, there is a requirement for the production of extremely pure water, including the removal of both dissolved and suspended or colloidal materials. One area where such a requirement is particularly important is in the steam generation of electrical power, in both fossil fuel and nuclear power plants. In both types of installations, it is common practice to include a filter bed in the recycle stream for the steam turbines.
A significant breakthrough in the purification of such liquid streams is described and claimed in U.S. Pat. Nos. 3,250,702 and 3,250,703, both of which are assigned to the assignee of this application. The invention described in these patents is based on the discovery that when finely divided anion and cation exchange resin particles are mixed in aqueous suspension, a volume increase is noted. This volume increase is the result of an agglomeration or "clumping" between the anion and cation exchange resin particles. Such resin particles, when used to form a filter bed, produce significantly reduced pressure drops across the bed, together with longer run lengths and improved efficiency of dissolved and undissolved solids removal.
As used herein, the term "bed" refers to a layer, such as a precoat layer, which has been deposited on a filter screen, an annular filter cartridge, a film, a deep or shallow bed, or the like. Such a bed may advantageously be deposited on a tubular filter cartridge.
A method for removing impurities from a liquid by passing the liquid through a filter bed which comprises a mixture of oppositely charged particles of filter aid material is described and claimed in U.S. Pat. No. 4,177,142, which patent is assigned to the assignee of this application. The filter aid materials are disclosed as including diatomaceous earth, cellulose fibers, charcoal, expanded pearlite, asbestos fibers, ion exchange resins, and inorganic ion exchangers. The filter bed comprises a mixture of oppositely charged particles of filter aid material. The filter aid particles normally have a surface charge in aqueous suspension. A portion of the particles is treated with a chemical compound to produce a surface charge which is opposite to the normal surface charge. A mixture of oppositely charged particles (normal and reversed) is therefore produced, and the "clumping" phenomenon is achieved.
A method for removing impurities from a liquid by passing the liquid through a filter bed which comprises treated filter aid material is mixed with finely divided ion exchange resin particles in the size range of sixty to four hundred mesh is described and claimed in U.S. Pat. No. 4,190,532. The patent is assigned to the assignee of this application. The mixture of treated filter aid material and ion exchange resin particles produces a clumping phenomenon. The filter aid materials include cellulose fibers, diatomaceous earth, charcoal, expanded pearlite, asbestos fibers and polyacrylonitrile fibers. The ion exchange resin particles include cation exchange resins, anion exchange resins or a mixture of both. The filter aid materials are treated with a chemical compound to produce the required surface charge.
A method for removing impurities from a liquid by passing the liquid through a filter bed which comprises a mixture of a treated fibrous filter aid material and an active particulate material is described in U.S. Pat. No. 4,238,334, which patent is assigned to the assignee of this application. The "fibrous filter aid materials" include cellulose fibers, polyacrylonitrile fibers, Teflon fibers, nylon fibers, rayon fibers, polypropylene fibers, and polyvinyl chloride fibers.
In U.S. Pat. No. 4,747,955 there is described a method for removing impurities wherein polyester fibers are used as a filter aid material in precoat filter formulations. The polyester fibers are treated with a hot caustic solution to convert the normally hydrophobic surface of the fibers to a hydrophilic surface.
In U.S. Pat. No. 4,313,832 there is described a method for purifying aqueous solutions by the use of ion exchange fibers. The ion exchange fibers have a thickness in the range of from about 2 to about 200.mu.m and a length of more than twice the thickness, to intertwine the fibers into a compact mass. The compact mass of fibers is applied as a slurry to a filter support to form a precoat layer thereon. The ion exchange fibers may be cation exchange fibers, anion exchange fibers or a mixture thereof. An alternative embodiment is disclosed wherein the intertwined ion exchange fibers have finely divided ion exchange resin particles of a diameter from about 2 to about 250.mu.m entrained between the ion exchange fibers.
It has been known to provide an overlay layer of ion exchange fibers onto a precoat layer of a mixture of ion exchange resins and cellulose fibers.
It has been found that precoat layers consisting of mixtures of ion exchange resin and inert fibers require a compromise between run length to a pressure drop end point and an iron oxide (crud) removal end point. It has also been found that precoat layers consisting of mixtures of ion exchange fibers and ion exchange resins have unsatisfactory crud removal.